As shown in FIG. 1, a hydraulic system for a construction machine in accordance with the prior art includes:
first and second hydraulic pumps P1 and P2 and a pilot pump, which are connected to an engine (not shown);
an arm cylinder 2 that is connected to a discharge flow path 1 of the first hydraulic pump P1;
a boom cylinder 4 that is connected to a discharge flow path 3 of the second hydraulic pump P2;
a first boom control valve 5 that is installed on an upstream side of the discharge flow path 3 of the second hydraulic pump P2 and is configured to be shifted to control a start, a stop, and a direction change of the boom cylinder 4;
a second boom control valve 7 that is installed on an upstream side of the discharge flow path 1 of the first hydraulic pump P1 and is configured to be shifted to allow a hydraulic fluid discharged from the first hydraulic pump P1 to join a hydraulic fluid supplied to the boom cylinder 4 from the second hydraulic pump P2 through a boom-up confluence flow path 6;
a first arm control valve 8 that is installed on a downstream side of the discharge flow path 1 of the first hydraulic pump P1 and is configured to be shifted to control a start, a stop, and a direction change of the arm cylinder 2;
a second arm control valve 10 that is installed on a downstream side of the discharge flow path 3 of the second hydraulic pump P2 and is configured to be shifted to allow a hydraulic fluid discharged from the second hydraulic pump P2 to join a hydraulic fluid supplied to the arm cylinder 2 from the first hydraulic pump P1 through an arm-in confluence flow path 9; and
a spool 12 (referring to a spool for controlling a pilot signal pressure to shift the second boom control valve 7) for the second boom control valve 7, which is configured to be shifted depending on whether an arm-in pilot pressure is higher or lower than a set pressure so that the second boom control valve 7 is shifted to a neutral position (i.e., a position “a”) if the arm-in pilot pressure is equal to or higher than the set pressure, and the second boom control valve 7 is shifted to a position (i.e., a position “b”) to allow the hydraulic fluid from the first hydraulic pump P1 to join the hydraulic fluid supplied to the boom cylinder 4 if the arm-in pilot pressure is lower than the set pressure.
A non-explained reference numeral 13 denotes a boom manipulation lever, and a non-explained reference numeral 14 denotes an arm manipulation lever.
A) The single boom-up operation will be described hereinafter.
In the case where the boom manipulation lever 13 is manipulated to ascend the boom, a spool of the second boom control valve 7 is shifted to the left on the drawing sheet by a boom-up pilot pressure partially applied to the position “b” of spool 12 for the second boom control valve. For this reason, the hydraulic fluid discharged from the first hydraulic pump P1 joins a hydraulic fluid of the boom-up confluence flow path 6 via the discharge flow path 1, the parallel flow path 15 of the first hydraulic pump P1, a check valve, and the second boom control valve 7 in this order. Simultaneously, a part of the boom-up pilot pressure causes a spool of the first boom control valve 5 to be shifted to the right on the drawing sheet. For this reason, the hydraulic fluid discharged from the second hydraulic pump P2 passes through the discharge flow path 3, the parallel flow path 16, the check valve, and the first boom control valve 5 in this order, and then is supplied to the boom cylinder 4 via the boom-up confluence flow path 6.
Thus, when the boom manipulation lever 13 is manipulated, the boom cylinder 4 can be driven to a boom-up state by the hydraulic fluids discharged from the first and second hydraulic pumps P1 and P2.
B) The single arm-in operation will be described hereinafter.
In the case where the arm manipulation lever 14 is manipulated to perform an arm-in operation, apart of the arm-in pilot pressure causes a spool of the first arm control valve 8 to be shifted to the right on the drawing sheet. For this reason, the hydraulic fluid discharged from the first hydraulic pump P1 is supplied to the arm cylinder 2 via the discharge flow path 1, the parallel flow path 15, the check valve, and the first arm control valve 8 in this order. Simultaneously, apart of the arm-in pilot pressure causes a spool of the second arm control valve 10 to be shifted to the left on the drawing sheet. For this reason, the hydraulic fluid discharged from the second hydraulic pump P2 passes through the discharge flow path 3, the parallel flow path 16 of the second hydraulic pump P2, the check valve, and the second arm control valve 10 in this order, and then joins is supplied to the arm cylinder 2 via the arm-in confluence flow path 9.
Thus, when the arm manipulation lever 14 is manipulated, the arm cylinder 2 can be driven to an arm-in state by the hydraulic fluids discharged from the first and second hydraulic pumps P1 and P2.
C) The combined boom-up and arm-in operation will be described hereinafter.
In the case where a combined operation is performed in which the manipulation lever 14 and the boom manipulation lever 13 are operated simultaneously to carry out a ground leveling work that flattens a ground surface, the arm-in pilot pressure according to the arm manipulation lever 14 causes the spool 12 for the second boom control valve 7 to be shifted to a position “a”. In other words, when the pilot pressure which shifts the spool 18 for the second boom control valve 7, is blocked to cause the spool 12 for the second boom control valve 7 to be shifted to a neutral position, the supply of the hydraulic fluid of the first hydraulic pump P1 to the boom-up confluence flow path 6 via the second boom control valve 7 is interrupted.
Thus, the boom cylinder 4 can be driven to a boom-up state only by the hydraulic fluid supplied from the second hydraulic pump P2.
Meanwhile, the boom-up pilot pressure according to the manipulation of the boom manipulation lever 13 causes the spool 17 for the second arm control valve 10 (referring to a spool for controlling the parallel flow path 16 connected to the second arm control valve 10) to be shifted to the top on the drawing sheet. For this reason, the supply of the hydraulic fluid discharged from the second hydraulic pump P2 to the arm cylinder 2 via the discharge flow path 3, the parallel flow path 16, the spool 17, the check valve, and the second arm control valve 10 is interrupted.
Thus, the arm cylinder 2 can be driven to an arm-in state only by the hydraulic fluid supplied from the first hydraulic pump P1.
The above conventional hydraulic system for a construction machine entails a problem in that when the ground leveling work is performed by the simultaneous operation of the arm manipulation lever 14 and the boom manipulation lever 13, it is not smoothly carried out due to a variation in a distribution of the hydraulic fluids of the first and second hydraulic pumps P1 and P2 and a load of the attachment such as a boom, or the like.
For this reason, in an attempt to solve the above problem involved in the conventional hydraulic system, a cross section of a spool notch for the arm-in and boon-up operation is made small to increase a load of the first and second hydraulic pumps (for example, the case where a load is applied to a return side to control the speed to be reduced), thereby improving manipulability. In this case, since the load of the first and second hydraulic pumps is increased due to the small cross section of the notch, the drive speed of the actuator becomes low and a pressure loss is increased to increase the amount of heat generated, thereby decreasing the fuel efficiency.